The utilities and principal power consuming industries, for an extensive period of time, have made use of transducers serving to evaluate or meter a broad variety of the parameters of measurement of electricity. Such parameters will include, inter alia, a.c. current, a.c. voltage, frequency, watts, vars, Q, watthours, varhours, Q hours, phase and power factor. When employed by industry, the transducers generally are provided having an input associated with power lines through isolation and scaling components, for example voltage and/or current transformers. Operating upon these inputs, the transducers then provided outputs, which, preferably, are present as linearly scaled currents which, for example, may be suitable as signals introduced to a data acquisition system such as a computer or some form of less elaborate recording instrumentation. Where the parameters are units of power, a multiplication must be carried out by the transducers, for example, watt monitoring signals represent the product of voltage and current.
Early technical approaches taken to develop power monitoring signals initially involved the use of thermally responsive coil elements and the like, the temperatures of which could be converted to outputs corresponding with power. A lack of convenience and accuracy with such techniques led to interest in the utilization of Hall effect devices as multiplers wherein a voltage-proportional generated magnetic field and current were associated to provide a voltage output proportional to the product of current and voltage. Transducers may also utilize an electronic arrangement serving to capitalize upon the exponential transfer characteristic of solid-state devices to carry out multiplication. However, for power monitoring applications these solid-state techniques exhibit an accuracy which is lower than desired.
Another technique for produce derivation currently popular in the industry utilizes the system concept of time division multiplication. For example, the multiplier produces a pulse waveform whose amplitude is proportional to one variable, whose length relative to period is a function of another variable, and whose average value is proportional to the product of the two variables. Early investigations of the use of electronic time division multipliers are reported upon in the following publications which are incorporated herein by reference:
I. E. A. Goldberg, "A High Accuracy Time Division Multiplier", RCA Review, Volume XIII, pp. 265-274, September, 1952. PA1 II. Sternberg, "An Accurate Electronic Multiplier", RCA Review, pp. 618-634, December, 1955. PA1 III. R. Bergeest and P. Seyfried, "Evaluation of the Response of Time-Division Multipliers to A.C. and D.C. Input Signals", IEEE. Transactions on Instrumentation In Measurement, Vol. 1 M-24, No. 4, pp. 296-299, December, 1975.
Time division multiplier networks generally are configured having a pulse width modulation circuit which is fed one of two input parameters and a switching circuit, usually controlled by the output signal of the modulator. As described in publication III above, the modulation circuits may be configured to operate in accordance with any one of three principles. In a first of these principles, the modulation factor is proportional to an input quantity. Accordingly, the pulse or sampling frequency is variable in accordance with input amplitude and an integration procedure is resorted to. In a second principle, the modulation factor remains proportional to an input parameter, but the sampling frequency does not depend upon input amplitude and thus is fixed. With such an arrangement, integration is not required. The third principle looks to the input parameter charging of capacitor associated with the utilization of a reference current discharge thereof and comparator detection. As in the second principle, no integration is carried out.
For any of the above principles elected by the designer of a transducer utilizing time division multiplication, the voltage input transformers inherently will evoke a phase shift of the incoming signal and the dictates of design accuracy require that such error be corrected. Further, where power factor related output signals are developed by the transducers, then a select phase shift, for example of 60.degree. to 90.degree. must be made available in the transducer design. Generally, the common technique for providing a phase shift correction, for example, into the integration stage of a transducer operating in accordance with the first principle, is to provide a separate compensation circuit which evokes a phase shift without a gain. While such phase shift correction can be achieved, the separate circuits involved have been observed to introduce their own errors into the entire transducer system design. Thus, difficulties conventionally encountered with amplifiers or the like, i.e. offset voltages, temperature occasioned excursions, and similar phenomena, are introduced with the result of loss of accuracy and reliability.